Field of the Invention
The present invention relates to an oxygen-barrier film, and to an adhesive used therefor.
Description of Related Art
Packaging materials used in the packaging of foodstuffs and beverages and the like must exhibit a wide range of functions, including not only functions such as strength, resistance to splitting, retort resistance and heat resistance that protect the contents during various types of distribution, storage such as refrigeration, and treatments such as heat sterilization, but also functions such as superior transparency that enable the contents to be viewed. On the other hand, when a bag is sealed by heat-sealing, an unstretched polyolefin film having excellent hot workability is essential, but unstretched polyolefin films exhibit many functions that are unsatisfactory as packaging materials.
For these types of reasons, composite flexible films composed of a combination of different polymer materials are widely used as the packaging materials mentioned above. These composite films are generally formed from an outer layer composed of a thermoplastic plastic film layer or the like that protects the merchandise and exhibits various functions, and a thermoplastic plastic film layer or the like that functions as a sealant layer. These layers may be bonded together using a method in which the thermoplastic plastic for the outer layer, an adhesive and the thermoplastic plastic for the sealant layer are subjected to three-layer melt extrusion to form an unstretched laminated sheet, and this laminated sheet is then stretched (for example, see Patent Document 1), but a dry lamination method in which an adhesive is applied to a laminate film layer, and the sealant layer is then bonded to the adhesive to produce a multilayer film (for example, see Patent Document 2) is simple to perform, and therefore widely used. However, the adhesive used for this application usually only performs the function of bonding the different films together.
Recently, even higher levels of functionality are being required of multilayer films, and oxygen-barrier properties that prevent the penetration of external oxygen in order to inhibit oxidation, carbon dioxide-barrier properties, and barrier properties relative to various odor components are now being demanded, with the purpose of enabling long-term storage of foodstuffs. When imparting barrier functions to a multilayer film, the unstretched polyolefin films typically used as the inner layer (on the sealant side) exhibit poor gas barrier properties, and imparting these films with barrier functions by coating or vapor deposition is difficult. As a result, the various films used for the outer layer (including polyester resins such as polyethylene terephthalate (hereafter abbreviated as PET), polyamide resins, and stretched polyolefin resins) are often imparted with barrier functions.
In those cases where coating is used to impart these outer layer films with barrier functions, one example of a widely used barrier coating material is vinylidene chloride, which exhibits superior oxygen-barrier properties and water vapor-barrier properties, but there are problems associated with the use of vinylidene chloride, including the generation of dioxin when the material is incinerated during disposal. Further, polyvinyl alcohol resins and ethylene-vinyl alcohol copolymers have also been used as barrier coating materials, but although these materials exhibit favorable oxygen-barrier properties under low humidity, they suffer from poor oxygen-barrier properties under high humidity, and inferior boiling resistance and retort resistance. On the other hand, films having a vapor deposited layer of a metal such as aluminum provided as a gas barrier layer are opaque, meaning the internal contents cannot be viewed, and are also unable to be used in microwave ovens. Further, films having a vapor deposited layer of a metal oxide such as silica or alumina provided as a gas barrier layer are expensive, and suffer from poor flexibility, resulting in a large variation in the gas barrier properties due to cracking and pinholes.
On the other hand, a method in which the adhesive used during lamination is imparted with oxygen-barrier properties is also known. This method offers the advantage that a barrier film can be produced even without using a special film that has been imparted with gas barrier properties by performing specific steps or employing a specific configuration during preparation of the laminated film. However, the type of flexible molecular structure that is required for an adhesive generally exhibits a high level of gas permeability. Accordingly, the adhesive performance and the barrier function often exist in a trade-off type relationship, and addressing this problem is technically difficult.
Examples of this type of gas barrier material that can be used as an adhesive for a laminated film include a resin described in Patent Document 3, which discloses a gas barrier composite film that uses a thermosetting bas-barrier polyurethane resin containing a resin cured product obtained by reacting an active hydrogen-containing compound with an organic polyisocyanate compound, wherein the resin cured product contains at least 20% by mass of skeletal structures derived from meta-xylene diisocyanate, and wherein within the active hydrogen-containing compound and the organic polyisocyanate compound, the proportion of compounds having three or more functional groups represents at least 7% by mass of the total mass of the active hydrogen-containing compound and the organic polyisocyanate compound.
However, this resin suffers from poor laminate strength with various films used as the aforementioned outer layer, and particularly when used with PET/CPP (cast polypropylene, hereafter abbreviated as CPP) film, which is widely used as a packaging for foodstuffs.
Further, Patent Documents 4 and 5 disclose the use of an epoxy resin cured product and/or a polyurethane resin cured product derived from meta-xylylene diamine as an adhesive that exhibits excellent oxygen-barrier properties.
However, these techniques require that the adhesive contains meta-xylylene structures derived from an expensive monomer in a high proportion (of at least 40% by mass, with some of the disclosed examples containing 50% by mass or more), and therefore a problem arises in that the packaging material becomes very expensive.